In recent years, various environmental contaminants emitted into the atmosphere from the internal combustion engines of power generators, vehicles, and the like have been viewed as a problem. Since the environmental contaminants are a main cause of acid rain and photochemical smog, they are also a major health hazard to human beings, and there has been action to globally regulate the emitted amounts thereof. In particular, with internal combustion engines such as diesel engines and lean burn engines of gasoline, since lean burn is performed, nitrogen oxides (hereinafter referred to as NOx) are emitted in a great amount, for example. Since purification of NOx and the like onboard a vehicle is not easy, progress has been made in investigating an efficient purification technique thereof.
For example, as a method of purifying NOx contained in the exhaust from an internal combustion engine in which lean burn is performed, a technique has been known in which NOx is temporarily absorbed or adsorbed via a NOx occlusion and adsorption catalyst under lean conditions in which the exhaust is oxygen excessive. With this technique, after NOx has been absorbed or adsorbed, a rich condition is produced in which the oxygen concentration is low by temporarily increasing the fuel injection amount, whereby the NOx thus absorbed or adsorbed is reduced.
For example, a technique in which a NOx absorption and purification catalyst made by combining an alkali metal such as potassium, an alkali earth metal such as barium and the like, and platinum and the like has been investigated (for example, refer to Non-patent Document 1, and Patent Documents 1 and 2). With this technique, first NOx is oxidized and absorbed on the NOx absorption and purification catalyst in the form of NO3−, using oxygen under the lean condition. Next, the oxygen concentration in the exhaust is reduced by controlling the internal combustion engine to a rich condition, and a state in which carbon monoxide and hydrocarbons exist in abundance is periodically formed, while the NOx thus absorbed under the lean conditions is catalytically reduced and purified with good efficiency using the carbon monoxide and hydrocarbons under the rich condition (under a reducing atmosphere).
In addition, with the above-mentioned technique as a basis, a technique has been investigated in which an hydrogen enrichment means disposed inside an exhaust passage and in which alkali metal and alkali earth metal are contained is provided upstream of a NOx absorption and purification catalyst composed of platinum and the like, and hydrogen-containing gas (hereinafter referred to as reformed gas) produced by this is introduced to the NOx absorption and purification catalyst disposed downstream (for example, refer to Patent Document 3). With this technique, reformed gas composed of hydrogen and carbon monoxide produced by the hydrogen enrichment means upstream is used in place of the hydrocarbons usually used as a reducing agent, during purification of NOx when rich.
In addition, as a technique resembling that of Patent Document 3, a technique has been investigated in which a hydrogen enrichment means provided separately from the exhaust passage of an internal combustion engine is operated independently from the internal combustion engine, and the reformed gas produced is introduced into the exhaust passage (for example, refer to Patent Document 4). Due to not influencing the operating conditions of the internal combustion engine such as the exhaust temperature, this technique can stably produce reformed gas to be a reducing agent of NOx compared to the technique of Patent Document 3.
A fuel reforming reaction using fuel is commonly used as the above-mentioned hydrogen enrichment means. The fuel reforming reaction is a reaction that uses hydrocarbons, which are fuel, to generate reformed gas containing hydrogen and carbon monoxide, and the reaction progresses at high temperatures of about 600° C. to 1200° C. The fuel reforming reaction is generally classified into two groups of partial oxidation reaction and steam reforming reaction according to the difference in oxidants.
A reaction that generates reformed gas containing hydrogen and carbon monoxide by way of a partial oxidation reaction is shown in reaction formula (I). This reaction is an exothermal reaction with fuel and oxygen as materials. As a result, the reaction progresses spontaneously, and thus a supply of heat from outside is unnecessary once the reaction is begun. However, in a case of fuel and oxygen coming to coexist in a high temperature state, a combustion reaction (complete oxidation reaction) as shown in reaction formula (II) also advances simultaneously on the catalyst. As a result, the catalyst temperature may rise above that intended, and thus control of the reaction is not easy.CnHm+½nO2→nCO+½mH2   (I)CnHm+(n+¼m)O2→nCO2+½mH2O   (II)
A reaction that generates reformed gas containing hydrogen and carbon monoxide by way of a steam reforming reaction is shown in reaction formula (III). This reaction is an endothermic reaction with fuel and water as materials. As a result, the reaction does not advance spontaneously, a result of which supply of heat from outside is essential, while control of the reaction is easy.CnHm+nH2O→nCO+(n+½m)H2   (III)
Patent Document 1: Japanese Patent No. 2586738
Patent Document 2: Japanese Patent No. 2600492
Patent Document 3: Japanese Patent No. 3642273
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2006-242020
Non-patent Document 1: “Development of NOx Storage Reduction Three-way Catalyst System,” Collective Papers of Society of Automotive Engineers of Japan, Vol. 26, No. 4, October 1995